Flutter DropdownButtonFormField未显示所选项目

Question

使用DropdownButtonFormField元素显示城市列表。它在更改事件上工作正常，并在更改时显示所选项目。问题是，一旦我尝试显示从设置状态中选择的项目，它将无法正常工作。尽管状态已成功设置并可以工作。

这是我的DropdownButtonFormField代码：-

              DropdownButtonFormField(
                value: _city,
                onChanged: (String newValue) {
                  setState(() {
                    _city = newValue;
                  });
                },
                // initialValue: 'Male',
                items: ['Ajman','Al Ain','Dubai','Fujairah','Ras Al Khaimah', 'Sharjah', 'Ajman','Umm Al Quwain']
                    .map((cityTitle) => DropdownMenuItem(
                    value: gender, child: Text("$cityTitle")))
                    .toList(),
              )

我在InitState中设置_city

@override
void initState() {
  setFilters();
  super.initState();
}

setFilters(){
   setState(() {
     _city = "Dubai";
   });
}

但是它没有显示选定的值。我该如何解决？

Answer 1

您可以在下面复制粘贴运行完整代码步骤1：移除重复的值'Ajman'步骤2：将输入错误value: gender更改为value: cityTitle

工作演示

完整代码

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  String _city;

  @override
  void initState() {
    setFilters();
    super.initState();
  }

  setFilters() {
    setState(() {
      _city = "Dubai";
    });
  }

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            DropdownButtonFormField(
              value: _city,
              onChanged: (String newValue) {
                setState(() {
                  _city = newValue;
                });
              },
              // initialValue: 'Male',
              items: [
                'Ajman',
                'Al Ain',
                'Dubai',
                'Fujairah',
                'Ras Al Khaimah',
                'Sharjah',
                'Umm Al Quwain'
              ]
                  .map((cityTitle) => DropdownMenuItem(
                      value: cityTitle, child: Text("$cityTitle")))
                  .toList(),
            ),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}